1. Field of the Invention
This invention relates to a communication system employing a spread spectrum pulse position modulation, and a transmitter and receiver for the communication employing the spread spectrum pulse position modulation, such as for indoor radio communication, radio LAN, and high-speed data communication.
2. Discussion of the Background
Japanese Patent Laid-Open Application No. 4-113732 discloses a spread spectrum pulse position modulation communication system. According to this spread spectrum pulse position modulation communication system, a one-cycle pseudo noise code is generated at each of regular intervals so that multi-valued information is sent at a start position of the one-cycle pseudo noise code, instead of sending information in accordance with a pseudo noise code swing, and this enables high-speed transmission.
Japanese Patent Laid-Open Application No. 4-137835 discloses a further spread spectrum pulse position modulation communication system, which improves upon the spread spectrum pulse position modulation communication system disclosed in Japanese Patent Laid-Open Application No. 4-113732. In this further spread spectrum pulse position modulation communication system, transmission data is subject to differential coding, so that a synchronizing pulse is eliminated, which thereby achieves further high-speed transmission.
In order words, the Japanese Patent Laid-Open Application No. 4-137835 discloses a spread spectrum pulse position modulation communication system which is capable of high-speed data communication within a limited band by using a one-cycle pseudo noise code for data transmission instead of a pulse of a pulse position modulating signal, and in which a start position of the pseudo noise code for modulation is made multi-valued, so that the communication can be speeded up more than any background direct-spread system for multiplying a primary modulation wave by a periodic pseudo noise code.
The spread spectrum pulse position modulation communication system disclosed in the Japanese Patent Laid-Open Application No. 4-137835 will now be described specifically with reference to FIGS. 13 to 15. As shown in FIG. 13(a), in this background spread spectrum pulse position modulation communication system, a pseudo noise (PN) code whose length is L is used for spread spectrum and the number of slots to be used for pulse position modulation is M, and one frame has M+L-1+j slots. Then one slot is selected from the M slots numbered from the start of the frame in correspondence to desired transmission data that has been subject to differential coding, and the pseudo noise code is inserted into L slots starting from the selected slot for spreading the transmission data. FIG. 13(b) shows a spread spectrum transmission signal in one frame of desired transmission data M' after being differential-coded at a second (2) slot position, where M=4, L=7, j=0. In this example, the pattern of the pseudo noise code is denoted as "1, 1, 1, -1, -1, 1, -1".
Referring next to FIGS. 14 and 15, circuit structures of transmission and reception units for data communication with such a frame structure of FIG. 13 will be described. As shown in FIG. 14, in a transmission unit a clock signal generator 1400 outputs clock signals to drive a pseudo noise signal generator 1401 and a counter 1402 whose value is reset to 0 after M+L-1+j counts. A serial/parallel converter 1403 converts serial data to be transmitted into parallel data having an M value and a register 1404 stores a last frame of the parallel data. An adder 1405 adds the current output data from serial/parallel converter 1403 and the last frame of parallel data stored in register 1404, and the added value of this data is fed back to the register 1404 and is differential-coded. Then, a comparator 1406 compares the output value of the register 1404 with the value of the counter 1402, and when both values match each other, comparator 1406 sends a trigger pulse signal to the pseudo noise signal generator 1401 to cause generation of a one-cycle pseudo noise code. It should be noted that the register 1404 is synchronized with a frame clock generated by a detector 1407 detecting when outputs of the counter 1402 reach a given value.
The clock generated by detector 1407 is multiplied by a phase lock loop PLL 1408 and the multiplied clock is used as a clock for converting serial data into parallel data in serial/parallel converter 1403. A multiplier 1410 multiplies a signal from the pseudo noise signal generator 1401 by a signal from an oscillator 1409 to convert the multiplied signal into a high-frequency signal to be transmitted as a radio signal from an antenna 1412 through a band pass filter 1411 and the like.
In a reception unit shown in FIG. 15, an antenna 1500 receives a signal from the transmission unit of FIG. 14. An amplifier 1501 amplifies the received signal and a multiplier 1503 multiplies the received signal by a local oscillation signal from an oscillator 1502 to convert the multiplied signal into an intermediate-frequency signal. The intermediate-frequency signal is amplified through a band pass filter 1504 and a gain control amplifier 1505, and is then inversely spread through a matched filter 1506 corresponding to the same pseudo noise code as used in the transmission unit of FIG. 14 so that a pulse position modulating signal is regenerated. The pulse position modulating signal is then detected by a detector 1507 and is converted into a base-band pulse position modulating signal. Then, a pulse interval measurement circuit 1508 following the detector 1508 measures a pulse interval of this signal and regenerates transmitted data from the measured value. Finally, a parallel/serial converter 1509 converts the transmitted data into serial data for regeneration of a signal identical to the input data signal of the transmitter of FIG. 14.
With such a background communication system, it is very difficult to achieve a high-speed transmission, and thus such a background system has not found use in a LAN, high-speed communication system, etc. In order to achieve a high-speed communication with such a background spread spectrum pulse position modulation communication system, a code division multiplexing is needed for spread spectrum communication. It is therefore necessary to employ a plurality of pseudo noise codes. These pseudo noise codes need to be spread through respective pseudo noise generators. This requires not only a transmitter to provide a plurality of pseudo noise generators, but also a receiver to provide a number of demodulation sections in correspondence to the number of pseudo noise generators, and this causes a problem in that the circuit structure is complicated.